Notifications feature proposal

[blakebarnett]

Currently to change the formatting or content of the slack notifications keel sends, it's necessary to modify the code and build/deploy a new image. Ideally this could be done via a ConfigMap that could be managed separately at runtime.

Switching to using notifications-engine might be a nice way to get these features and a bunch more options. Though it would require some fairly big changes.

I wouldn't mind working on this if it's something that would get accepted.